Automatic idle time keying unit



fume E6, i936. G, USSELMAN AUTOMATIC IDLE TIME KEYING UNIT Filed Feb.17, 1934` 5 Sheets-Sheet 1 INVENTOR G. L. USSLMAN ATTORNEY G. 1 UssELMAN2,044,539

AUTOMATIC IDLE TIME KEYING UNIT Filed Feb. 17, 1954 5 sheets-sheet 2 J le i6, 3936.

june 3.5, 1936. G.- L USSELMAN 2,044,539

AUTOMATIC IDLE TIME KEYING UNIT Filed Feb. 17, 1954 5 Sheets-Sheet 5ATTORN EY G L USSELMAN AUTOMATIC IDLE TIME KEYING UNIT ATTORNEY PatentedJune 16, 1936 PATET AUTOMATIC' EDLE TME KEYING UNIT Application February.17, 1934, .Serial No. 711,764

'15 Claims.

This invention relates to .telegraphy signalling means and in particularto keying control switching for radio transmitters, said keying controlswitching means being automatic in operation.

Transmitters of this type in general comprise a line for incomingcontrol or keying impulses or source of keying impulses, and atransmitter adapted to signal at different frequencies, including anoscillation generator, the necessary amplifiers and radiating meanscoupled therewith, and keying means interposed between the signal lineor source and one or more stages `in the transmitter. The signals fromthe source, or on the line from the remote point, key the transmitter tosend out signals.

In transmitters of this type vknown in the prior art it has been thepractice to send out Vs or "a-b-d between messages in order ,that vthereceiving apparatus destined to receive said messages may be kept intune and ready for the next message. This continued operation of thetransmitter in sending out these Vs or a-b-c is wasteful of powerbecause these idle signals used up as much power as is used by signalsin a useful message. One of lthe .objects of the present invention is toeliminate or at least reduce materially this waste of power.

Briey, attain the object Ymentioned above by the use of a novel devicefor automatically switching the transmitter over to a source of signalsof relatively short duration. These signals require less power totransmit and serve the same purpose as the Vs or a-b-c of normal lengthtransmitted heretofore. The control impulses may be Vs or any otherdesired characters oi reduced length. These signals may have widerspaces as well as shorter dots and dashes. For example a dot or dashone-third as long and spaces that much longer may be used and theutilization or dots and dashes of reduced length and spaces of increasedlength will require only about one-third as much power as is required tosend out the control or tuning impulses sent out heretofore at normalspacing.

The source of Vs or other characters of 4reduced length and theincreased spaces may be sent out by means of a wheel. The characters maybe arranged on the periphery or on the face oi the whe-el which whenArotated will in accordance with the present invention transmit the dotsand dashes of reduced length and the spaces oi increased length. Thewheel may also include call letters or character-s which identify theparticular channel on which signalling is going on. The wheel will forconvenience be termed a V (Cl. Z50-17) wheel throughout kthisspecication. It will be understood, however, that other means may beused 4tofsend out the signals of reduced length and spaces of increasedlength without departing from the spirit of the present invention. Ofcourse 5 each transmitter may send out signals-on several channels, andthe frequencies of the waves used .on the various channels may bedifferent. In my .inventionl provide a V wheel as described above foreach channel, and the V wheels may be thrown in or out of circuit, byswitching means which is operated simultaneously with and by the deviceused to change the frequency of the signals worked with.

Where the keying control is operative during extended no-signal periodsas described above, the telegraph line may become open-circuited orinoperative for some other reason without the operator at the .receiverend being aware of Vsuch in- ,operativeness The open circuit wouldmerely 20 connect the selected V wheel to the keying tube to send voutthe Vs or other short pulses for tuning purposes and the identifyingsignals.

vAnoloject of the present invention is to provide :means whereby theoperator at the receiving station will Yknow whether the operator at thetransmitter .is still in control and when such control is relinquished.This is accomplished in accordance with the present invention byproviding a simplied means whereby the transmitter may be disconnectedfrom the channel andconnected to one of the sources of Vs or othercharacters of reduced length and spaces of increased length by thepresence of a continued marking character on the line, and disconnectedfrom the V wheel 1 and connected to the message line in the absence ofsaid continued marking current.

Briefly the object mentioned above is accomplished in accordance withapplicants Ynovel .invention by interposing the contacts of a relaybetween the V wheels and the keying apparatus in the transmitter andinterposing a thermionic relay between the source ofsignal and thearmature in said relay as in the prior modications and operating saidthermionic device in such a manner that as long as normal messagecharacters separated by normal spaces or intervals are coming in overthe line or yfrom the source the transmitter will be co-nnected withsaid line or source, vbutas soon as a signal character of -continuousduration is put on the line the transmitter Will be disconnected fromthe message or line and connected to the selected V wheel. This actiontakes place slowly so that there is no danger of the transmitter beingdisconnected 55 from the message line or source as long as messages arecoming in. When messages are again started the thermionic relay isactuated to permit the contact of the relay to close the circuit betweenthe message source or line and the tran,.- mitter. This operation takesplace quickly so that the transmitter is connected to the message lineor channel immediately so that no messages are mutilated.

The novel features of my invention have been set forth in detail in theclaims appended hereto.

The nature of. my invention and the manner in which the same may becarried out will be best understood by the detailed description thereofwhich follows and therefrom when read in connection with the attacheddrawings throughout which like reference characters indicate like partsinsofar as possible and in which:

Figure l illustrates a circuit arrangement including the essentialelements of my novel automatic idle time keying unit. In this circuitthe essential elements only of the transmitter keyed by my novel circuitare shown;

Figure 2 shows modification of the arrangement of Figure 1, While;

Figures 3, 4, and 5 show a modification or" the arrangement of Figure 2.In the circuits of Figures 3, 4, and 5 however, the operativeness of thetransmitter to send signals or tuning impulses is determined by thepresence or absence `of signals or a pulse of continuous duration on theline.

Referring to all of the gures, the keying unit is indicated at A, athermionic relay which controls the mechanical relay is indicated at B,the mechanical relay is indicated at Z, the transmitter keyed stage isindicated at E, the transmitter keying stage is indicated at D and thesource of characters of. reduced length and increased spaces for tuningpurposes at the receiver and channel identifying symbols is indicated atC by the V Wheels v1 and v2.

In Figure 1 keying unit A has its output connected to the control grid2`of thermionic tube V1 and also to contact I3 of relay Z as shown. Thecathode 3 of tube V1 is grounded. The anode I of tube V1 is connected tothe positive end of a source B1, while the negative end of B1 isconnected to the grid 5 of tube V2, and also to ground by way of aresistor R1. The cathode 6 of tube V2 is connected to ground as shown.The anode 4 of tube V2 is connected to the positive terminal of a highvoltage source HV by way of resistor R4 and to the grid 8 of tube V3 andto one terminal of a condenser I0, the other terminal of which isconnected to ground. The cathode 9 of tube V3 is connected to thepositive terminal of a source B2, the negative terminal of which isgrounded. The anode II of the tube V3 is connected to the coil I I ofthe relay Z and by way of the coil I I of the relay Z to the positiveterminal of a high potential source. All of the high potential sourcesmentioned above may be replaced by a single source when desired.Furthermore this source may be obtained from a generator or by way ofrectifiers from an alternating current source. The tongue I2 of relay Zis connected to the grid electrodes of a pair of thermionic tubes V4 andV5 as shown in the keying stage D of the transmitter. A spring I5retains the tongue I2 of, relay Z pulled over against contact I3 whenthere is no current flowing in coil II. Contact I4 is connected to abrush on one of the V Wheels. To simplify the description of applicantsdevice it will be assumed that wheel V2 is connected in circuit bycontact K. The other brush on V wheel Z is connected as shown to asource of negative potential. Contacts' I3 and Ie are also connected toa source of 10W positive potential through resistors Rs and RF1. Thetongue I2 is connected to a low positive voltage by Way of a resistor R7of high value.

The stage D, that is the keying stage of the transmitter, and the stageE of the transmitter form no part of the present invention. However, itmay be said that the tubes V4 and V5 of stage D and the tube Vs of stageE all derive their anode voltage and current by way of a resistance R5from any source of high voltage HV. If the control grids of tubes V4 andV5 are maintained at positive potential or a small negative potentialtubes V4 and V5 are conductive and draw anode current. In this mannerthey divert current and potential from the anode electrode of thetransmitter tube in E and decrease the amplitude of the oscillationsproduced by E. In this manner the transmitter is keyed. The thermionictube E may be an amplifier, a frequency multiplier or a relay and mayhave its control electrode excited or energized by a source ofoscillations as shown. The output circuit of E may be coupled to autilization circuit directly or by way ot ampliers or frequencymultipliers or both. The utilization circuit may be an aerial system ora transmission line.

In the operation of Figure 1 it will rst be assumed that there is nosignal being sent from the telegraph oice to the vacuum tube keying unitat A, which may be located at or near the transmitter. Under thesecircumstances the output voltage of unit A is zero and the grid 2 oftube V1 is made positive by the current through Re. The tube V1 is thenconductive and current flows through the resistance R3. Because of thedrop of current in the resistance R3 the grid 5 of. tube V2 becomessufciently negative to out off the ow of anode current in V2. Thispermits the condenser IU to charge up slowly to a higher positive valuethrough resistance R4 which is connected to the high voltage source HV.When the charge in condenser Ill has reached a suiiiciently high valuethe grid 8 of tube V3 Will be made suciently positive in respect tocathode 9 to cause current to flow in the anode circuit thereof. Thiscurrent passing through coil II of relay Z produces a magnetic eld whichattracts the armature I2 to overcome the bias supplied by the spring i5and to pull the armature I2 over to make contact with the contact pointI4. The V wheel V2 is now connected to the control grids of the keyingtubes V4 and. V5 in the transmitter and when said V Wheel is rotatedcurrent impulses representative of the signal on the periphery thereofor on the face thereof are impressed on the control grid of the keyingtubes V4 and V5 for transmission.

The V wheel operates as follows:

When the brush I6 passes over the conducting segment, on the rotating VWheel, negative voltage is impressed on the grids of tubes V4 and V5,which causes these tubes to be non-conductive. The anode potential oftube Ve is sufficiently high to allow this tube to function as anamplifier or a relay or as a frequency multiplier so that carrier energyis impressed on the following stage and passes on through thetransmitter to the antenna or transmission line. Likewise, when brush Iil passes over a non-conducting segment on the V wheel, the grids of;tubes V4 and Vs of stage D become slightly positive because of thecurrent through resistor Rs. This causes tubes' V4 and V5 to conductvcurrent which is drawn through resistor R5. This extra current causes amuch: greater voltage drop in R5 so that the anode; potential of tube:Ve in stage E is too low to conduct suiicient current and the carrierenergy is cut oi from the. remaining stages of the transmitter and nosignal is transmittedV at this moment. The characters onthe V Wheel maybe of any desired length and may as indicated above be very short whilethe spaces may be of greatly increased length.

Now assume that a signal or message comes in over the telegraph line.This. message causes keying unit A to send out a series of negativepotential. pulses. The first negative pulse cuts oi the current in tubeV1 and the,- grid 5 of tube'Vz changes toground. or cathode potentialbecause there is no current flowing in. Ra.- This causes tube Va tobecome conductive so that current flows. in the anode circuit thereof.Con-denser l0 quickly discharges. by way olthev path of low resistancein tube V2, which lowers the potential on the grid 8 of tube V3 withrespect to the cathode 9 thereof. The tube V3 immediately becomesnon-conductive thereby deenergizing the winding ll of relay Z. Thisallows the field of the magnet to fall and permits the-spring I5 to pullthe tongue i2 over against contact I3 which connects the transmitterkeying stage D to the vacuum tube keying device A. Keying isaccomplished in the-same manner described in connection with the Vwheel. The negative impulses of high potential from the output of A,biases the control grids of Vl andVtV negatively so that their anodecircuits are non-conductive. This causes a potential on the anode of Vsto increase and transmission of oscillations takes place. When thepotential from A to the control grids of V4 and V5 is zero or lessnegative the keying tubes become conductive thereby lowering thepotential on the anode of Vs to cut off oscillations in the transmitter.If the signals from A should stop for, say, ten seconds then condenserI0 will have time to charge up sufficiently to cause the control grid oftube V3 to become positive enough to start a flow of current in theanode circuit of V3. This current energizes H and actuates relay Z toconnect the transmitter keying stage D to V wheel V2.

The overall operation would be that as long as the telegraph ofce issending the transmitter will respond, but if the telegraph ofce stopssending for several minutes or for aV period longer than a pre-arrangedtime delay, the transmitter will be idle until the time delay is up andthen the V wheel will be automatically switched on. This is where thesaving in power occurs. The V wheel dots and dashes are shorter thannormal, while the intervals between dots and dashes are longer thanusual.

This requires less transmitter power than normal keying requires and yetserves to keep the receiving stations tuned to the transmitterfrequency. The change from the V wheel to the message line control isalmost instantaneous, but a comparatively long period is requiredtoautomatically switch from the message channel to the V Wheel.

The V wheel may be made so that any ratio of length between dots andspaces can be obtained by moving the brush near to or further from thecenter. The number of dots or spaces per time intervalv maybe controlledby controlling the speedofi rotation oi the wheel.

theA modification. ofY Figure 2.1 have climi.- nated the necessityof;the separate; sources B1 B2, To; accomplish thisY l1 interpose aresistance R2 between the anode of V1 and the control grid of V2.Positive potential for the anode of V1 is Asupplied byY way of aresistance R1 from any source of: high potential. Biasing potential forthe control grid ot V2 may besupplied by way of a resistance. Rsconnected to a negative point on asource. The. current for the cathodeofVa may be supplied from an alternating current source andi the:cathode 9 of; V3` may beconnected to a source of positive potential.Figure 2 otherwise operates in the same manner in which the.modification of Figure l operates as described in detailhereinbefore.For this reason. it is thought unnecessary to repeat the op eration ofthe circuits at this point. howeventhat'the action of. Vrcauses throughthe networkV of resistors R1 Rz, and Ra-potential changes on the grid 5of V2 as in Figure l.

When it is desired. to control the relation between the transmitter andthe message channel or the V Wheel from. the message source the keyingdevice may be made. to function from an extended dash put on the messageline at the source or telegraph oiiice. When the keying control is sooperated the receiving operator is conx scious of the fact that thetelegraph operator still has the transmitter under his control. If theautomatic keying control is made to function by the extended no-signalperiod, as in the .prior modifications, the telegraph control linerangement of Figure 2 disclosed hereinbefore.

The operation of this device is the same as the operation of the priorarrangements except as indicated hereinafter.

At the end of a message or group of messages -the telegraph operatorsends a prolonged dash over the line to the vacuum tubekeying unit A.This causes the vacuum tubekeying unit A to maintain a. steady negativepotential in its output. rihegridl 5 of tube V2 is in this case madesuciently negative to block the anode current in the tube. Condenser l0then slowly charges through resistor R4 andcauses the grid 8 of tube V3to become suiciently positive with respect to the cathodef whichv isalready positive to some extent topermitanode current to flow in V3.This anode current passes through the coil Il of the relay Z tocause therelay tongue I2 to make contact withv contact I4 which connects the se--lected V wheel. to the control grids of the tubes V4 and V5 of thetransmitter keying unit D. As long as the dash is maintained on thetelegraph control line. the V wheel will continue to key thetransmitter. l

Now if a telegraph message is started the long dash istaken oi thecontrol line. No voltage appears on the output of the vacuum tube keyingunit AY and the grid 5 of tube V2 assumes practically ground potentialbecause of its connection to ground through resistor Ra. TubeVz quicklydischarges.condenserv l0.- andalso draws. more cur- The modification ofIt is noted,

rent from the positive source through the resistor R4, so that thepotential drop in this resistor R4 prevents further building up ofpositive potential on condenser I9 and grid 8 of tube V3. This causesgrid 8 to be sufciently negative with respect to the cathode 9 of thistube to block the anode current. Consequently relay Z is deenergized andthe relay tongue I2 is pulled over by spring I5 to make contact withcontact I3. This in eilect connects the keying stage D of thetransmitter to the vacuum tube keying unit A. When the next dot or dashcomes, condenser I starts to charge slowly through resistor R4 butbefore it is charged is suciently high to cause relay Z to operate, thenext space between signals causes it to quickly discharge through tubeV2 so that as long as signals or prolonged nosignal periods aremaintained the transmitter keying stage will remain connected to thesignal line or telegraph channel.

Very briey, when the telegraph operator wishes to put on the economy Vshe holds a continuous dash on the control line. After a certainadjustable time delay the automatic control device switches thetransmitter keying control to the V wheel. When the operator wishes tostart a message he may start immediately and the automatic controldevice very quickly `switches the transmitter keying control to thetelegraph control system.

In a modification the automatic keying control device described abovemay be used on the extended dash while the extended no-signal period maybe used through another and somewhat similar control system to stop andstart the transmitter.

The apparatus may be simplied in practice. For example, I may utilizethe arrangement shown in Figure 4. Here the cathodes of the tubes V2 andV3 may be both heated by alternating current supplied from anyalternating current source. Furthermore, the secondary windings of theseheating circuits need not be centertapped but may be grounded at oneside as shown. Cathode 6 may be maintained at the desired potential byconnecting one side of the heating circuit thereof to the terminal ofresistance R3 which is grounded. The desired positive potential for thecathode 9 of tube V3 may be obtained by connecting one side of theheating circuit for cathode 9 to the tap on a potentiometer P connectedas shown between the positive terminal of the direct current source andground which is also connected to the negative terminal of the source ofdirect current. Here, as in Figure 3, the charge on the condenser If! isdetermined by the amount of current flowing in the anode circuit of tubeV2 and in turn determines the bias of the control grid 8 of tube V3. Theconductivity of tube V3 determines whether or not the winding II ofrelay Z is energized. 'I'he keying of the transmitter by the V wheel orby messages from A is accomplished here in the same manner in which itis accomplished in Figure 3` and it is thought unnecessary to describein detail the operation of the arrangement in Figure 3. The V wheels V1and V2 here as in the prior modifications may be rotated in any manneras for example by means of a motor. The motor in turn may be energizedin any manner and may be energized as shown by alternating current fromthe same source used to energize the cathodes of the tubes V2 and V3.

In arrangement of Figure the output of the vacuum tube keying device Ais connected to contact I3 and to oneterminal of Rs, the other terminalof which is grounded. The electrical center oi' the heating circuit ofV2 is connected to ground while the electrical center of the cathodeheating circuit of V3 is connected to a point on a potentiometer Pconnected between a high potential source and ground. The lowerpotential N of potentiometer P may be shunted by a capacity as shown andin the operation of the arrangement shown in Figure 5 as in the priormodications the conductivity of V3 is determined by the charge ofcondenser I0 which is in turn determined by the conductivity of V2.

The transmitters of each modification are usually provided with wavechanging devices in one or more stages to permit signalling on aplurality' of different frequencies or channels. Each channel iskpreferably identified by its own call letters or symbols. In myinvention I provide a V wheel for each channel. Each V wheel may carrychannel identifying letters or symbols in addition to the Vs or tuningsignals. The desired channel may be selected by a wave change switch WC.VThe particular V wheel that carries the identifying letters for theselected channel may be thrown in circuit by a switch VS which may beconnected to the control for the wave change switch WC as shown.

Having thus described my invention and the operation thereof, what Iclaim is:

l. A signalling system comprising a transmitter of the thermionic typeincluding a source of signais, keying means interposed between saidsource of signals and said transmitter, and means for sending outcontrolling signals of reduced length between messages, comprising, acontrol signal producing device, a relay, and thermionic means coupledto said source of signals on the one hand and to said relay on the otherhand, said ther- -mionic means being responsive to the presence orabsence of signals in said source to disconnect said source from saidtransmitter between messages and to connect said control signalprodu-cing device to said transmitter.

2. A signalling system comprising, a transmitter including a thermionickeying tube, a source of signals, a source of control impulses, a relayincluding contacts for connecting said keying tube to said source ofsignals or said source of control impulses, and a thermionic deviceinterposed between said source of signals and actuating means in saidrelay, said thermionic device responding to a constant signal from saidsignal source to actuate said relay to connect said source of controlimpulses to said keying tube. l

3. A signalling system comprising, a transmitter including a thermionickeying tube, a source of signals, a source of impulses of short durationseparated by intervals of long duration, means for connecting saidkeying tube to said source of signals or said source of short impulsesincluding, a thermionic relay device interposed between said source ofsignals and said keying tube, said thermionic device responding to aconstant signal from said signal source to actuate said relay to connectsaid source of short impulses to said keying tube.

4. A signalling system comprising, a transmitter of the thermionic type,a source of signals, keying means interposed between said source ofsignals and said transmitter, and means for sending out controllingsignals of reduced length sepa.- rated by spaces of increased lengthbetween messages, comprising, a control signal producing device, amechanical relay including a movable contact connected to said keyingmeans, and xed contacts connected to said source of Signals and' Y insaid source to move said movable contact against one or the other oisaid fixed contacts.

5. In a signalling system, a transmitter of the therrnionic type, asource of signals, a thermionic keying tube connected with saidtransmitter, a

` magnetic relay having a movable contact and a pair oi xed contacts, aconnection between said keying tube and one of said contacts, a sourceof control' signals connected with another of said contacts, aconnection between the remaining one of said contacts and said source ofsignals, a

thermionic tube having output electrodes con# nested with the energizingwinding in said relay, said tube having input electrodes, and a couplingbetween the input electrodes of said thermionic tube and said signalsource.

6. A signalling system comprising, a transmitter of the thermionic type,a source of signals, keying means interposed between said source ofsignals and said transmitter, and means for sending out controllingsignals of reduced length between messages comp-rising a controllingsignal producing device, a mechanical relay, and thermionic meanscoupled to said source of signals on the one hand and to said mechanicalrelay on the other hand to disconnect said source of signals from saidtransmitter between messages and to connect said controlling signalproducing device to said transmitter, said operations taking placeslowly and means for quickly disconnecting said transmitter from saidcontrol signal device and connecting the same to said source of signals.

7. In a signalling system, a transmitter of the thermionic typeincluding a keying tube, a source of signals, a wheel having conductingsegments connected to a source of potential and separated bynon-conducting segments, a relay having a movable member connected tosaid keying tube and a pair of contacts, a connection between a brush onsaid wheel and one of said contacts, a connection between the other ofsaid contacts and said source of signals, a thermionic tube having anoutput connected with an energizing winding in said relay, said tubehaving an input circuit, and a coupling between the input circuit ofsaid thermionic tube and said signal source.

8. A signalling system comprising, a transmitter of the thermionic type,a source of signals, a keying tube interposed between said source ofsignals and said transmitter, and means for sending out controllingsignals of reduced length and increased spaces between messagescomprising a controlling signal producing device, a mechanical relayhaving an armature connected to said keying tube, said relay having apair of contacts and a winding, a connection between said source ofsignals and one of said contacts, a connection between said controllingsignal producing device and the other of said contacts, thermionic meanscoupled to said source of signals on the one hand and to said winding onthe other hand to disconnect said source of signals from saidtransmitter when signals from said source are interrupted and to connectsaid control signal source to said transmitter said operations takingplace slowly and for quickly disconnecting said transmitter from saidcontrol signal source and connecting the same to said source of signalswhen signals are resumed.

9. A signalling device comprising, in combina-A tion carrier waveproducing and keying means,l

wave length changing means `connected therewith, a source of messages, aplurality of sources of short impulses and wave length identifyingsymbols, a device for selecting one of said last named sources, commoncontrol means for sai-dwave length changing means and said sourceselecting device, and a relay for connecting either said source ofmessages or said selected short impulse and wave length identifyingsymbol source to the keying means in said transmitter.

iQ. A signalling system comprising, a wave `re-V laying circuit of thethermionic type, wave changing means connected to the relaying circuit,a source oi signals, a thermionic keying tube connected with saidrelaying circuit, a relay having a plurality of contacts, a connectionbetween one of said contacts and said keying tube, a plurality oisources of control signals and wave length identifying symbols, a switchfor selecting one of said sources and connecting the same to another ofsaid contacts, a connection between athird one of said contacts and saidsource of signals, therinionic means connected with an energizingwinding in said relay, a coupling between the input circuit of saidthermionic means an-d said signal source, and a common control for saidswitch and said wave changing means.

l1. A signalling system comprising, carrier wave relaying means and wavelength changing means, a source of signals, keying means interposedbetween said source of signals and said carrier wave relayings means,means for sending out controlling signals of reduced length separated byspaces of increased length between messages and wave identifyingsymbols, comprising, a plurality of control signal producing devices,there being a device for each wave length, means for selecting aparticular one of said devices, a mechanical relay including a movablecontact connected to said keying means, and xed contacts connected tosaid Source of signals and to the selected one of said devices,thermionic means coupled to said source of signals on the one hand andto said mechanical relay on the other hand, said thermionic means beingresponsive to the presence or absence of signals'in said source to movesaid movable contact against one or the other of said fixed contacts,and a common control for said wave length changing means and said deviceselecting means.

12. In a signalling system, a wave relaying circuit, wave changing meansconnected to said relaying circuit, a source of signals, a thermionickeying tube having a plurality of electrodes, a connection between apair of said electrodes in said keying tube and said relaying circuit, amagnetic relay having an energizing winding and a plurality of contacts,a connection between one of said contacts and one of the electrodes insaid thermionic keying tube, a plurality oi sources of control signalsand wave length identifying symbols, a switch for selecting one of saidlast named sources and connecting the same to another of said contacts,a connection between a third of said contacts and said rst named sourceof signals, a thermionic tube having input and output electrodes,resistive means connecting the input electrodes of said thermionic tubeto said source of signals, an additional thermionic tube having inputand output electrodes, resistive and capacitive elements coupling theinput electrodes of said additional thermionic tube to the outputelectrodes of said preceding thermionic tube, and a circuit connectingthe output electrodes of said additional tube in series with saidenergizing winding of said relay.

13. In a signalling system, a wave relaying circuit, a source ofsignals, a keying tube having a plurality of electrodes, a connectionbetween a pair of the electrodes in said keying tube and said relayingcircuit, a magnetic relay, having an energizing winding, a movablecontact and a pair of xed contacts, a connection between said movablecontact and one of the electrodes in said keying tube, a source ofcontrol impulses, means connecting the source of control impulses to oneof said fixed contacts, a connection between the other of said xedcontacts and said source of signals, a control tube having input andoutput electrodes including a cathode, an impedanace connecting theinput electrodes of said control tube to said source of signals, animpedance cori-i nected between said fixed contacts, a connectionbetween a point on said impedance and the cathode of said control tube,an additional tube having input and output electrodes, an impedance anda capacity coupling the input electrodes of said additional tube to theoutput electrodes of said control tube, and a circuit connecting theoutput electrodes of said additional tube with said energizing windingof said relay.

14. A signalling system as recited in claim 13 wherein all of saidimp-edances are resistive in character.

15. A system as recited in claim 13 in which the input and outputelectrodes of said additional tube includes a cathode and in which thecathodes of said control tube and said additional tube are 15 heated byalternating current.

GEORGE LINDLEY USSELMAN.

